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Modified files:
books/developers-handbook/x86 chapter.sgml
Log:
Kapitel 12.12 uebersetzt.
Gesendet von: Fabian Borschel
ToDo: die screen und programlisting -bloecke muessen gefixt werden.
Revision Changes Path
1.9 +1588 -1753 de-docproj/books/developers-handbook/x86/chapter.sgml
Index: chapter.sgml
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RCS file: /home/cvs/de-docproj/books/developers-handbook/x86/chapter.sgml,v
retrieving revision 1.8
retrieving revision 1.9
diff -u -I$FreeBSDde.*$ -r1.8 -r1.9
--- chapter.sgml 29 Aug 2007 19:37:48 -0000 1.8
+++ chapter.sgml 30 Aug 2007 18:35:50 -0000 1.9
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</sect1>
<sect1 id="x86-fpu">
-<title>Using the <acronym>FPU</acronym></title>
-<para>
-Strangely enough, most of assembly language literature does not
-even mention the existence of the <acronym>FPU</acronym>,
-or <emphasis>floating point unit</emphasis>, let alone discuss
-programming it.
-</para>
-
-<para>
-Yet, never does assembly language shine more than when
-we create highly optimized <acronym>FPU</acronym>
-code by doing things that can be done <emphasis>only</emphasis> in assembly language.</para>
-
-<sect2 id="x86-fpu-organization"><title>Organization of the <acronym>FPU</acronym></title>
-<para>
-The <acronym>FPU</acronym> consists of 8 80–bit floating–point registers.
-These are organized in a stack fashion—you can
-<function>push</function> a value on <acronym>TOS</acronym>
-(<emphasis>top of stack</emphasis>) and you can
-<function>pop</function> it.
-</para>
-
-<para>
-That said, the assembly language op codes are not <function role="opcode">push</function>
-and <function role="opcode">pop</function> because those are already taken.</para>
-
-<para>
-You can <function>push</function> a value on <acronym>TOS</acronym>
-by using <function role="opcode">fld</function>, <function role="opcode">fild</function>,
-and <function role="opcode">fbld</function>. Several other op codes
-let you <function>push</function> many common
-<emphasis>constants</emphasis>—such as <emphasis>pi</emphasis>—on
-the <acronym>TOS</acronym>.
-</para>
-
-<para>
-Similarly, you can <function>pop</function> a value by
-using <function role="opcode">fst</function>, <function role="opcode">fstp</function>,
-<function role="opcode">fist</function>, <function role="opcode">fistp</function>, and
-<function role="opcode">fbstp</function>. Actually, only the op
-codes that end with a <emphasis>p</emphasis> will
-literally <function>pop</function> the value,
-the rest will <function>store</function> it
-somewhere else without removing it from
-the <acronym>TOS</acronym>.
-</para>
-
-<para>
-We can transfer the data between the
-<acronym>TOS</acronym> and the computer memory either as
-a 32–bit, 64–bit, or 80–bit <emphasis>real</emphasis>,
-a 16–bit, 32–bit, or 64–bit <emphasis>integer</emphasis>,
-or an 80–bit <emphasis>packed decimal</emphasis>.
-</para>
-
-<para>
-The 80–bit <emphasis>packed decimal</emphasis> is
-a special case of <emphasis>binary coded
-decimal</emphasis> which is very convenient when
-converting between the <acronym>ASCII</acronym>
-representation of data and the internal
-data of the <acronym>FPU</acronym>. It allows us to use
-18 significant digits.
-</para>
-
-<para>
-No matter how we represent data in the memory,
-the <acronym>FPU</acronym> always stores it in the 80–bit
-<emphasis>real</emphasis> format in its registers.
-</para>
-
-<para>
-Its internal precision is at least 19 decimal
-digits, so even if we choose to display results
-as <acronym>ASCII</acronym> in the full
-18–digit precision, we are still showing
-correct results.
-</para>
-
-<para>
-We can perform mathematical operations on the
-<acronym>TOS</acronym>: We can calculate its
-<emphasis>sine</emphasis>, we can <emphasis>scale</emphasis> it
-(i.e., we can multiply or divide it by a power
-of 2), we can calculate its base–2
-<emphasis>logarithm</emphasis>, and many other things.
-</para>
-
-<para>
-We can also <emphasis>multiply</emphasis> or
-<emphasis>divide</emphasis> it by, <emphasis>add</emphasis>
-it to, or <emphasis>subtract</emphasis> it from,
-any of the <acronym>FPU</acronym> registers (including
-itself).
-</para>
-
-<para>
-The official Intel op code for the
-<acronym>TOS</acronym> is <varname role="register">st</varname>, and
-for the <emphasis>registers</emphasis>
-<varname role="register">st(0)</varname>–<varname role="register">st(7)</varname>.
-<varname role="register">st</varname> and <varname role="register">st(0)</varname>, then,
-refer to the same register.
-</para>
-
-<para>
-For whatever reasons, the original author of
-<application>nasm</application> has decided to use
-different op codes, namely
-<varname role="register">st0</varname>–<varname role="register">st7</varname>.
-In other words, there are no parentheses,
-and the <acronym>TOS</acronym> is always
-<varname role="register">st0</varname>, never just <function role="opcode">st</function>.
-</para>
-
-<sect3 id="x86-fpu-packed-decimal">
-<title>The Packed Decimal Format</title>
-<para>
-The <emphasis>packed decimal</emphasis> format
-uses 10 bytes (80 bits) of
-memory to represent 18 digits. The
-number represented there is always an
-<emphasis>integer</emphasis>.
-</para>
-
-<tip>
-<para>
-You can use it to get decimal places
-by multiplying the <acronym>TOS</acronym>
-by a power of 10 first.
-</para>
-</tip>
-
-<para>
-The highest bit of the highest byte
-(byte 9) is the <emphasis>sign bit</emphasis>:
-If it is set, the number is <emphasis>negative</emphasis>,
-otherwise, it is <emphasis>positive</emphasis>.
-The rest of the bits of this byte are unused/ignored.
-</para>
-
-<para>
-The remaining 9 bytes store the 18 digits
-of the number: 2 digits per byte.</para>
-
-<para>
-The <emphasis>more significant digit</emphasis> is
-stored in the high <emphasis>nibble</emphasis>
-(4 bits), the <emphasis>less significant
-digit</emphasis> in the low <emphasis>nibble</emphasis>.
-</para>
-
-<para>
-That said, you might think that <constant>-1234567</constant>
-would be stored in the memory like this (using
-hexadecimal notation):
-</para>
-
-<programlisting>
-80 00 00 00 00 00 01 23 45 67
-</programlisting>
-<para>
-Alas it is not! As with everything else of Intel make,
-even the <emphasis>packed decimal</emphasis> is
-<emphasis>little–endian</emphasis>.</para>
-
-<para>
-That means our <constant>-1234567</constant>
-is stored like this:
-</para>
-
-<programlisting>
-67 45 23 01 00 00 00 00 00 80
-</programlisting>
-<para>
-Remember that, or you will be pulling your hair out
-in desperation!
-</para>
-
-<note>
-<para>
-The book to read—if you can find it—is Richard Startz'
-<ulink url="http://www.int80h.org/cgi-bin/isbn?isbn=013246604X">8087/80287/80387
-for the IBM PC & Compatibles</ulink>.
-Though it does seem to take the fact about the
-little–endian storage of the <emphasis>packed
-decimal</emphasis> for granted. I kid you not about the
-desperation of trying to figure out what was wrong
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Received on Thu 30 Aug 2007 - 20:37:15 CEST